leetcode -- Merge k Sorted Lists add code

Merge k sorted linked lists and return it as one sorted list. Analyze and describe its complexity.

[解题思路]

以前的解法的时间复杂度过高，通过在网上搜索，得到优化的时间复杂度：O(n*lgk)

维护一个大小为k的最小堆，每次得到一个最小值，重复n次

 1 /**
 2  * Definition for singly-linked list.
 3  * public class ListNode {
 4  *     int val;
 5  *     ListNode next;
 6  *     ListNode(int x) {
 7  *         val = x;
 8  *         next = null;
 9  *     }
10  * }
11  */
12 public class Solution {
13     public ListNode mergeKLists(ArrayList<ListNode> lists) {
14         // Start typing your Java solution below
15         // DO NOT write main() function
16         ListNode head = null;
17         int len = lists.size();
18         if(len <= 1)
19             return null;
20         head = merge2List(lists.get(0), lists.get(1));
21         for(int i = 2; i < len; i++){
22             head = merge2List(lists.get(i), head);
23         }
24         return head;
25         
26     }
27     
28     public ListNode merge2List(ListNode node1, ListNode node2){
29         ListNode head = null;
30         ListNode tmp = head;
31         while(node1 != null && node2 != null){
32             if(node1.val <= node2.val){
33                 ListNode node = new ListNode(node1.val);
34                 tmp = node;
35                 tmp = tmp.next;
36             } else {
37                 ListNode node = new ListNode(node2.val);
38                 tmp = node;
39                 tmp = tmp.next;
40             }
41             node1 = node1.next;
42             node2 = node2.next;
43         }
44         
45         while(node1 != null){
46             ListNode node = new ListNode(node1.val);
47             tmp = node;
48             tmp = tmp.next;
49             node1 = node1.next;
50         }
51         
52         while(node2 != null){
53             ListNode node = new ListNode(node2.val);
54             tmp = node;
55             tmp = tmp.next;
56             node2 = node2.next;
57         }
58         return head;
59         
60     }
61 }

上一版本有bug，修复如下：

 1 public class Solution {
 2     public ListNode mergeKLists(ArrayList<ListNode> lists) {
 3         // Start typing your Java solution below
 4         // DO NOT write main() function
 5         ListNode head = null;
 6         int len = lists.size();
 7         if(len == 0)
 8             return null;
 9         else if(len == 1){
10             return lists.get(0);
11         }
12         head = merge2List(lists.get(0), lists.get(1));
13         for(int i = 2; i < len; i++){
14             head = merge2List(lists.get(i), head);
15         }
16         return head;
17         
18     }
19     
20     public ListNode merge2List(ListNode node1, ListNode node2){
21         ListNode head = new ListNode(Integer.MIN_VALUE);
22         ListNode tmp = head;
23         while(node1 != null && node2 != null){
24             if(node1.val <= node2.val){
25                 ListNode node = new ListNode(node1.val);
26                 tmp.next = node;
27                 tmp = tmp.next;
28                 node1 = node1.next;
29             } else {
30                 ListNode node = new ListNode(node2.val);
31                 tmp.next = node;
32                 tmp = tmp.next;
33                 node2 = node2.next;
34             }
35         }
36         
37         if(node1 != null){
38             tmp.next = node1;
39         }
40         
41         if(node2 != null){
42             tmp.next = node2;
43         }
44         head = head.next;
45         return head;
46         
47     }
48 }

http://blog.csdn.net/zyfo2/article/details/8682727

http://tech-wonderland.net/blog/leetcode-merge-k-sorted-lists.html